Other than trying to keep his head up i'm not sure tbh!
Just try to keep his head up, or if you feel his back tense up, turn him tight or disengage his quarters, though most horses are quick and you won't really have time to before he does it..
I tend to just sit to them and ride through it personally , if it's just a lighthearted, excited buck, or go "oiiii ! " lol, maybe a little tap on the shoulder with your crop if really needed.
Don't think that's really helped has it?
Hope you sort it!